FIGS. 11 and 12 illustrate the ranges of power supplied to a slave device in an old specification and a new specification, respectively.
In recent years, USB (Universal Serial Bus) has become prevalent as an interface for connecting two devices. For example, various devices may be connected to a personal computer by USB, such as a printer, a digital camera, a mobile phone unit, a floppy disk drive, a portable audio-player, a PDA (Personal Digital Assistant), and a USB memory.
According to the USB, data is exchanged between two devices, and also power is supplied from a host device (a higher-level device; typically a personal computer or a USB hub) to a slave device (a lower-level device). Therefore, with the use of the USB, a secondary battery provided in the slave device may be charged by supplying power from the host device.
Non-patent document 1 defines specifications for supplying power to a slave device by the USB.
In a specification (old specification) previous to this specification (new specification), power needs to be supplied by the VBus from the host device to the slave device within the range of the “existing region” indicated in FIG. 11.
However, with the new specification, it is possible to supply power within the range of the “extended region” indicated in FIG. 11. Alternatively, when data communications are also performed between the host device and the slave device, it is also possible to supply power within the ranges of the “high speed extended region” and the “full speed/low speed extended region” indicated in FIG. 12, according to the transfer mode. As described above, in the new specification, restrictions relevant to supplying power are mitigated.
Non-patent document 1: “Battery Charging Specification Revision 1.0”, May 8, 2007, USB Implementers Forum, Inc.
However, because the restrictions have been mitigated as described above, there are cases where the slave device does not receive power supplied as expected. When the slave device does not receive power, the slave device may not normally execute some processes.
For example, assuming that the slave device does not recognize being connected to the master device unless the slave device receives a voltage of four volts or more, the slave device does not operate properly when a voltage of only two volts is applied to the slave device.